MSKP capture: Don't force skiaglthreaded

The original intent was to avoid using GLESRenderEngine, since MSKP
capture cannot be used with that backend. Moreover, now we have
skiavkthreaded, and we should be able to capture in that mode as well.

Noticed while investigating b/296282988, though this is not directly
related.

Bug: 296282988
Change-Id: I5ab4a180a3649bfc536dcee82e6a699f165a3555